Influences of pH and moisture content on the methane production in high-solids sludge digestion

The effects of pH and moisture content on high-solids sludge digestion were investigated using a mesophilic batch digester fed with sludge cake. The experiments were carried out by changing the initial moisture contents from 90 to 96% and the initial pH from 5.0 to 10.0. A simple model developed from the Gompertz equation was applied to estimate the methane production rate and the lag-phase time under various conditions, based on the cumulative methane production curves. The relative methanogenic activity decreased with the decrease of moisture content and dropped from 100 to 53% when the moisture content decreased from 96 to 90%. The rate of the methane production of the high-solids digestion at moisture contents of 90 to 96% functioned over a range of 6.6-7.8 with an optimum of pH 6.8, whereas the process may fail if the pH is lower than 6.1 or higher than 8.3. Moreover, a minimum lag-phase time for methane production was also found at around pH 6.8. In addition, a modified Haldane equation was suitable to represent the effect of pH on methanogenic activity at various moisture contents. The maximum specific methane production rate, K-m, and the saturation rate constants for hydrogen ion, K-H, and hydroxyl ion, K-OH, were 14.5-7.5 ml CH.g(-1) dry weight.d(-1), 5.0 x 10(-1)-8.0 x 10(-1) [M] and 2.0 x 10(-9)-8.0 x 10(-9) [M], respectively. (C) 1997 Elsevier Science Ltd.